2. Materials and methods
2.1. Martials
All reagents used were supplied by Sigma-Aldrich or Merck company without further purification. L-methionine amino acid (Merck, 99.0 %), tetran-butylorthotitanate (TBOT, C₁₆H₃₆O₄Ti, Merck, 98.0 %), ethanol (C2H5OH, Merck, 99.0 %), acetic acid (CH3COOH, Merck, 99.8 %), zinc acetate dihydrate (Zn(CH3COO)2.2H2O, Sigma-Aldrich, 99.99 %) and diethanolamine (NH(CH₂CH₂OH)₂, 99.5 %) were used to synthesize C, N, S triple doped TiO2-ZnO nanocomposites. Direct red 16 (C26H17N5Na2O8S2, Alvan Co., Iran) was used an organic pollutant. Deionized water was prepared by..... . Chemical substances which were used for COD experiments were supplied by Merck company. Biologically treated POME was collected from effluent of…show more content… Among the different RSM experimental designs, the Full face-cantered central composite design (CCD) was employed. CCD is one of the excellent tools to establish RSM model. It is a flexible and efficient model of RSM that was provided the maximum amount of data on the effects of parameters with a minimum number of experiments (PCR, RSM 10).
First, the appropriate variables and the range of selected variables were determined by preliminary experiments. In the case of Direct red 16, four numerical variables including dye concentration, initial pH (3, 7 and 11) photocatalyst concentration, and irradiation time were selected to design photodegradation experiments. Table 2 showed 30 run conditions (three series of experiments) including 16 factorial points, 8 axial points and 6 central points based on following equitation (1):
N 2k-qk nc (1)
Where k, q, and nc are the number of factors, fraction of the number factor and the replicate number of the central point, respectively. For full factorial design q is zero. Also, Table 2 presents obtained experimental results at each…show more content… Fig.2 depicts the performance of various photocatalysts (pure TiO2, C, N, S triple doped TiO2, TiO2-ZnO, C, N, S triple doped TiO2-ZnO nanoparticles with different mass ratios of L-methionine and ZnO) under visible light. Pure TiO2 did not exhibit much activity under visible light. It is obvious that, the optimum weight fraction of L-methionine was achieved at 1.5 wt % with .....dye removal efficiency. Similar findings as reported in the literature confirmed the optimum amount of dopants not only reduced TiO2 band gap but also inhibited photoinduced electron and hole recombination [1ZnOmix]. Therefore, the optimum dopant concentration was improved photocatalytic activity. But when the amount of L-methionine as a doping agent of C, N and S is more than 1.5 wt%, the photocatalytic activity was abated due to the detrimental effect of the excessive doped C, N, S atoms. The incorporated C, N, and S atoms into the TiO2 lattice act as a new recombination center of photoinduced e+/h pairs and reduce separation efficiency of photogenerated carriers at 3 wt% of L-methionine (p1, Ni doped-TiO2-ZnO). Also, it was proposed that the Direct red 16 removal was increased with increasing ZnO to TiO2 mass ratio up to 1:3 from .....to.......due to the difference of redox energy levels between ZnO and TiO2 and improve charge separation and avoid the